1. Field of the Invention
The present invention relates to measuring apparatuses, and more specifically to an improvement in their fine-shape detection mechanism.
2. Prior Art
Fine-shape measuring apparatuses such as scanning probe microscopes have been used conventionally for measuring the fine shapes of workpieces. One type of scanning probe microscope is an atomic force microscope, for instance. The atomic force microscope detects an atomic force acting between the surface of a workpiece and a stylus, and the unevenness of the surface of the workpiece can be detected by scanning the surface of the workpiece in such a manner that the detected atomic force is kept constant (Japanese Unexamined Patent Application Publication No. 2002-181687).
In the fine-shape measuring apparatuses, the displacement of a stylus can be measured in many ways. One example is an optical lever method (Japanese Unexamined Patent Application Publication No. 2002-181687, Japanese Unexamined Patent Application Publication No. Hei-9-72924) utilizing a laser and an optical position-sensitive detector. The measurement may be made by a focus error detection method (Japanese Unexamined Patent Application Publication No. Hei-9-61441). The measurement may also be made by an optical fiber interferometer (Japanese Unexamined Patent Application Publication No. Hei-7-301510, Japanese Patent Publication No. 3081979).
An example of the optical lever method is shown in FIG. 1 of Japanese Unexamined Patent Application Publication No. 2002-181687. In this method, a laser beam is directed to the rear surface of a cantilever, and reflected light is detected by an optical position-sensitive detector located away from the cantilever. When a stylus of the cantilever is displaced up and down in accordance with the unevenness of the surface of the workpiece, the amount of displacement is reflected as an angular change of reflected light. In the optical lever method, the angular change is magnified at a distance and detected by the optical detector. The angular change of the reflected light is detected from a light intensity signal received by the optical detector, and the height of the surface of the workpiece, indicated by the stylus of the cantilever, can be measured accordingly.
In the fine-shape measuring apparatuses, the height of the surface of the workpiece can be measured by scanning the surface of the workpiece and moving the surface of the workpiece relatively up and down in such a manner that the light intensity signal of the optical detector keeps a certain value. Through the detection of the height of the surface of the workpiece as described above, the fine-shape measuring apparatuses can measure the fine shape of the workpiece along a single line or in a two-dimensional area.
The fine-shape measuring apparatuses must measure much finer shapes than those measured in general shape measurement, and therefore, very-high-accuracy measurement is required.
A variety of measuring apparatuses have been proposed to meet this requirement. For instance, the one disclosed in Japanese Unexamined Patent Application Publication No. Hei-7-301510 has a normal trihedral reflector in a probe for the purpose of reducing the adverse effect of torsion in a guide mechanism or the like on the measurement accuracy. A technique suggested in this publication detects the amount of displacement of the probe by measuring the amount of displacement of the normal trihedral reflector by means of a plurality of displacement gauges.
The conventional methods, however, do not provide a sufficient measurement accuracy and have room for further improvement.
The inventors have studied this problem and have found the following: In the measurement performed by scanning on the workpiece, data obtained by a sensor for sensing the height are the sum of the amounts of up-and-down movements of the cantilever in accordance with the shape of the workpiece and the amounts of up-and-down movements of the cantilever, caused by motion errors produced by a scanner.
One conventional method to reduce these measurement errors is to use a high-accuracy scanner with low motion errors. However, it is very difficult to prepare an ideal scanner that is free from motion errors. Even if an ideal scanner could be prepared, it would be very expensive, making it difficult to manufacture an economical apparatus.
Accordingly, the present invention does not use a very-high-accuracy scanner as a means for solving the problem. Another method suggested to eliminate the effect of motion errors produced by the scanner requires a complicated apparatus utilizing a normal trihedral reflector.
Therefore, in the fine shape measurement field, there has been strong demand for the development of a technology that can implement high-accuracy measurement without using a very-high-accuracy scanner. However, no conventional technology has met the demand appropriately.